Hitherto, an optical disc has been widely used as a recording medium for recording and/or reproducing an information signal by radiating a light beam on an information-recording surface thereof.
Such an optical disc known in the art includes an optical disc of a ROM (read only memory) type used only for reproducing an audio signal or an image signal already recorded thereon or reading out an information signal such as data signals, an optical disc of a DRAW (direct read after write) type on which an information signal is recordable in an unerasable manner only one time, a magneto-optical disc of a type capable of repeatedly recording and erasing an information signal thereon, or the like.
The optical disc of the ROM (read only memory) type includes a substrate made of a transparent material such as a synthetic resin or glass. Formed on a primary surface of the substrate as a signal-recording surface, are pits which are fine irregularities corresponding to the audio signal or other data signals recorded. The signal-recording surface of the substrate is covered with a reflecting film layer made of aluminum or the like.
Further, the magneto-optical disc capable of repeatedly recording the information signals also includes a substrate made of a transparent material such as a synthetic resin and glass, on a primary surface of which a vertically magnetized thin film layer having magneto-optical properties such as a Kerr effect or a Farady effect is formed. The vertically magnetized thin film layer is also covered with a reflecting film layer made of aluminum or the like. The vertically magnetized thin film layer of the magneto-optical disc, which serves as a signal recording layer, has been made of an amorphous alloy of a rare-earth element and a transition metal such as a Tb--Fe--Co alloy and the like.
Furthermore, the optical disc of a DRAW (direct read after write) type has a substrate made of a transparent material such as a synthetic resin and glass. Formed on a primary surface of the substrate is a signal-recording film layer composed of a low-melting metal thin film layer, a phase-changing film layer, an organic dye-containing film layer, or the like. In such an optical disc of the DRAW (direct read after write) type, the signal-recording film layer is also covered with a reflecting film layer made of aluminum, or the like.
Meanwhile, a huge number of optical discs have been used as a recording medium for an information signal-recording and/or reproducing apparatus. Therefore, it is desirable that the optical discs are produced with a high mass productivity and a low manufacturing cost. In order to meet such requirements, the substrate for the optical disc is obtained and widely used in the form of an injection-molded article made of a synthetic resin such as a polycarbonate resin.
The substrate used for the optical disc of the ROM type is formed with data pits which are fine irregularities corresponding to the audio signal or the image signal recorded. The recording tracks are formed on the substrate in a concentric or spiral relation to a center of the substrate of the optical disc. In the recording tracks, there are formed prepits which generate tracking error signals for controlling a scanning position of the light beam to be radiated to the surface of the optical disc, when detected.
Further, in the magneto-optical disc capable of repeatedly recording the information signals or the optical disc of the DRAW (direct read after write) type, the substrate is formed thereon with prepits which are in the form of fine irregularities arranged along virtual recording tracks disposed concentrically or spirally relative to the center of the substrate of the optical disc.
In the optical disc of the ROM (read only memory) type, it is required to accurately scan the recording track by the light beam serving for reading out the information signal so that the information signal recorded on the optical disc can be exactly reproduced. Further, in order to perform an exact recording of the information signals on the magneto-optical disc or the optical disc of the DRAW (direct read after write) type, the light beam for recording the information signals is required to exactly scan the recording tracks on which the information signals are to be recorded.
Consequently, in the case of the recording and/or reproducing apparatus in which the optical disc of the ROM (read only memory) type or the optical disc capable of recording the information signals is used as a recording medium, a tracking control is carried out to control a position of the light beam relative to the recording track on the optical disc so as to cause the light beam to exactly follow the recording track. For the purpose of such a tracking control, the prepits are formed on the optical disc and detected by the light beam so that a tracking control signal capable of controlling the scanning position of the light beam is generated. On the basis of the detection output of the tracking control signal, a position of the objective lens is controlled such that the light beam radiated is focussed or converged onto the aimed recording track of the optical disc.
Incidentally, a sampling servo system is known as a system for realizing the tracking control in which the light beam radiated is controlled so as to exactly follow the recording track of the optical disc. In the optical disc used in the sampling servo system, the recording tracks on which the audio signal or other information signals are to be recorded are divided into a plurality of data areas in each of which two prepits are formed. The prepits generate a tracking control signal for controlling the scanning position of the light beam when detected. As shown in FIG. 1A, the prepits 11a and 11b are provided on the opposite sides of a center line C of a recording track formed on the optical disc in such a manner that these prepits are spaced at a predetermined distance from each other in the scanning direction of the light beam. The two prepits 11a and 11b are so arranged as to be offset relative to the center line C in such a manner that they sandwich the center line C of the width of a virtual recording track. Specifically, the two prepits 11a and 11b are in the form of irregularities each located at a position which is spaced at a distance of one-fourth of the width of the recording track apart from the center line C in the radial direction of the optical disc, namely in the direction perpendicular to the center line C of the recording track. Hereinafter, the prepit 11a which is located rearward relative to the scanning direction of the light beam and therefore first radiated by the light beam when scanned in the direction indicated by an arrow S in FIG. 1A, and the prepit 11b which is radiated by the light beam after the prepit 11a is radiated by the light beam, are referred to as "the first prepit" and "the second prepit", respectively.
As mentioned above, in the tracking control in which the two prepits 11a and 11b formed on the optical disc are employed, the returning light beams reflected on the first and second prepits 11a and 11b are detected by a detecting device to obtain the respective detection outputs which are then compared with each other whereby a tracking error signal is obtained. Thereafter, the objective lens driving device is operated on the basis of this tracking error signal to control the movement or displacement of an objective lens through which the light beam to be radiated is focussed or converged onto the recording track. As a result, the tracking control is performed such that the light beam radiated through the objective lens onto the optical disc can follow the recording track exactly.
At this time, in the case where a beam spot SP of the light beam to be converged through the objective lens and radiated on the optical disc, is scanning along the center line C of the recording track, the detection output levels obtained from the first and second prepits 11a and 11b are identical, as shown in FIG. 1B. On the other hand, if the beam spot SP of the light beam is scanning along a path offset from the center line C of the recording track to a side near the first prepit 11a, the detection output level obtained from the first prepit 11a is higher than that obtained from the second prepit 11b. To the contrary, if the beam spot SP of the light beam is scanning along a path offset from the center line C of the recording track to a side near the second prepit 11b, the detection output level obtained from the second prepit 11b is higher than that obtained from the first prepit 11a. Accordingly, a deviation, i.e., a tracking error of the beam spot SP relative to the recording track can be detected by obtaining the difference between the detection output levels from the first and second prepits 11a and 11b. A tracking error signal generated by the detection of the tracking error controls an operation of the objective lens driving device so that the objective lens thereon is displaced so as to cause the light beam to be radiated onto the optical disc to follow the recording track exactly. Thus, the tracking control can be performed in a proper manner.
In the sampling servo system, the prepits formed on the optical disc are used as a reference for determining a clock phase which is used at a time when an information signal recorded on the recording track should be reproduced. Consequently, in the case of the optical disc used in the sampling servo system, in order to cause the light beam to scan the recording track exactly and enable an exact read-out (reproduction) of the information signal recorded on the recording track without omission thereof, it is required to exactly form the prepits each having a given size at given positions on the optical disc.
Meanwhile, as described hereinbefore, the substrate constituting the optical disc is prepared by the injection-molding of a synthetic resin material such as a polycarbonate resin having a light-permeability.
The substrate made of the synthetic resin is produced by injecting a melt of the synthetic resin material such as a polycarbonate resin into a die in which there is disposed a stamper having a pit-pattern reverse to shapes of the prepits or the data pits corresponding to the information signals to be recorded on the optical disc. In this case, the die is maintained in a closed state. Next, the die is compressed while cooling so that the pit-pattern of the stamper is transferred to the synthetic resin material injected thereinto. After curing the melt of the synthetic resin material injected into the die, the die is opened to release the cured synthetic resin material from the stamper whereby the substrate for the optical disc is obtained.
In the above-mentioned molding process of the substrate, there occurs a risk that the transferred pit-pattern formed on the cured article deviates from those on the stamper due to thermal shrinkage of the synthetic resin material of the substrate upon curing.
That is, when the substrate is prepared from a synthetic resin material by using an injection molding process, the melt having an elevated temperature upon its injection into the die undergoes a decrease in its temperature when the die is cooled for curing so that a shrinkage force is caused in an interior of the shaped substrate. This stress or shrinkage force significantly occurs at a region between the pit-pattern of the stamper and prepits correspondingly formed on the shaped substrate. The stress concentrated at the region between the pit-pattern of the stamper and the prepits correspondingly formed on the substrate causes a deviation between the pit-patterns of the stamper and the prepits formed on the substrate when the molded substrate is released from the die. That is, the pit-pattern of the stamper cannot be exactly transferred to the molded substrate so that the transferring deviation of the pit-pattern occurs.
When the transferring deviation occurs, the respective prepits 11a and 11b, which are not consistent with the pit-pattern of the stamper, are continuously formed with deformed portions 11c and 11d as shown in FIG. 2A. The deformed portions 11c and 11d are of such a shape that one side of each prepit is stretched toward the right side thereof when viewed along the scanning direction S of the beam spot SP. Once such the deformed portions 11c and 11d are formed in the first and second prepits 11a and 11b due to the transferring deviation upon molding, there occurs a difference between the detection output levels of the first and second prepits 11a and 11b as shown in FIG. 2B even though the beam spot SP is scanning exactly along the center line C of the recording track on which the information signals are recorded. In the case that a center position of the recording track should be determined by the difference in the detection output levels between the first and second prepits 11a and 11b, the center position of the recording track is erroneously detected as if it is located on a wrong center line C' which deviates from the inherent center line C, as shown in FIG. 2A. As a result, an exact tracking control for scanning of the recording track by the light beam cannot be achieved.
Thus, owing to the transferring deviation of the first and second prepits 11a and 11b from the pit-pattern of the stamper, the clock phase used upon reproduction of the data signals recorded on the optical disc suffers from undesired phase shift so that recording of the information signals on the optical disc and/or reproduction thereof from the optical disc cannot be exactly performed.